The present invention relates to an apparatus for mixing solids with one another and with liquids, particularly for producing foundry moulding sand mixtures, comprising a circular, fixed trough with a vertical axis and at least two groups of mixing tools fixed to arms, which revolve with a relative speed and preferably in opposite directions with a different spacing from the trough axis and from the trough bottom and are so inclined with respect to the tangent of their circular path or orbit under opposite angles, that the mixing tools of the outer group feed the material being mixed inwards and the mixing tools of the inner group feed the material being mixed outwards.
In an apparatus of the aforementioned type (U.S. Pat. No. 4,154,540), the drive of the mixing tool groups is located in the vicinity of the trough axis, each group of tools having its own drive. The mixing paddles or blades are connected by means of supporting arms to the driving shaft. Gear connections are optionally provided between the supporting arms. This known mixer, which is particularly intended for mixing building materials, e.g. concrete mixtures, has an inner and an outer group of mixing tools with the same rotation direction revolving over the trough bottom and another group of mixing tools revolving in the opposite direction in the overlap region thereof and above the same. The mixing tools are simple plates, positioned perpendicularly to the trough bottom, which are set at an angle with respect to the tangent of their circular path or orbit. Due to a differing setting of the mixing tools, the material being mixed is conveyed from both the outer and inner regions of the trough in the direction of the overlapping region of the two first mixing tool groups and there is carried in the opposite direction by the mixing tool group located above it.
Compared with mixers with mixing tools revolving in only one direction or with stationary tools and a revolving trough, the aforementioned mixer has the major advantage that the material being mixed is not only alternately moved from the outside to the inside and vice-versa, but is also exposed to a constant shear load. Thus, homogeneous mixing in the case of a high flow rate is achieved for a large number of starting components.
However, this leads to considerable problems in the case of foundry moulding sands in that in addition to the quartz sand used as the main part, there are various solid and usually pulverulent aggregates and liquid cementing or binding agents with different viscosities which have to be processed. The individual solid and liquid components have widely differing specific gravities, so that within the mixer layers having different component enrichment or concentration levels are formed within the mixer due to sedimentation movements and it is very difficult to break them up. Unlike in the case of concrete mixtures, in the case of moulding sand mixtures, each individual particle must be surrounded by a cementing agent layer, which on the one hand requires a corresponding contact time between the cementing agent and the moulding sand particles and on the other hand requires corresponding forces, e.g. frictional forces, which lead to the pressing of the cementing agent onto the particles. These functions can not or can only be inadequately fulfilled by a mixer of the aforementioned construction. It has also been found that with increasing viscosity, the material being mixed does not sufficiently rapidly drop from the mixing tools again and is instead carried along over a longer distance, so that the more viscous the mixture, the longer the mixing time. Finaly, the filling level and therefore the throughput per mixing cycle is limited by the mixing tools only revolving in two planes.
Thus, in the past, mixer having a special construction have been developed, which takes account of the special circumstances connected with foundry moulding sand. Thus, it is known (DE-C-1 204 632) to arrange different mixing and kneading or rolling tools within a revolving trough, which are driven about axes running parallel to the trough axis, partly in opposite and partly in same directions. As a result of this construction, the mixing components are subject to different movements and forces, which is intended to lead to a good mixing quality. In another known mixer (U.S. Pat. No. 3,964,733), radially arranged mixing arms having paddles revolve over the bottom of the stationary trough and convey the material being mixed in the circumferential direction and also raise the same by setting counter to the rotation direction. Above said mixing arms, centrifugal tools rapidly rotating eccentrically to the axis engage in the trough, constantly tear apart the upper layer of the material being mixed and centrifuge the latter in opposite directions. This mixer also leads to a satisfactory mixing quality, but requires considerable driving forces and leads to a high degree of wear.
It is finally known (DE-A-1 557 252) to use a plurality of planar mixing tools revolving about vertical axes in a revolving trough and to move the material being mixed from the outside to the inside by stationary deflector plates. In addition, mixing tools are provided having on a vertical axis a plurality of spaced, superimposed, planar paddles, which impart a vertical movement to the material being mixed. Thus, in this mixer horizontal and vertical movements are superimposed in the material being mixed. It is disadvantageous in this construction and in many of those described hereinbefore, that the differently acting mixing tools always require their own drive, so that accessibility from above to the trough is considerably impaired. Due to the asymmetrical arrangement of the mixing tools dead spaces or gaps are formed, in which the material being mixed is only exposed to a particular component of force or motion, so that the different effects on the material being mixed occur at different geometrical locations and there is no true superimposing of said effects. Thus, the residence time is correspondingly long and the mixer volume inadequately used.
The invention is initially based on the known fact that an optimum mixing quality, particularly in the case of foundry moulding sands, requires both horizontal and vertical components of motion within the material being mixed. In addition, the mixture is to be exposed to constant shear loads, in order to effectively counteract the formation of agglomerates.